The development of new materials and processes for the production of metal pieces with the aim of reducing component weight at a low cost, which is of utmost importance in the automotive industry. In order to achieve these objectives, the industry has developed ultra-high-strength steels (UHSS) which exhibit an optimized maximal strength per weight unit and advantageous formability properties. These steels are designed to attain a microstructure after heat treatment, which confers good mechanical properties and makes them especially suited for the hot stamping process used to form steel blanks into particular automobile parts. Since during the hot stamping process the blank is subjected to aggressive atmospheres, the steel is usually coated to avoid corrosion and oxidation.
In an attempt to minimize the weight of components while respecting structural requirements, so-called “tailored blank” techniques may be used. In these techniques, components may be made of a composite metal blank, which is obtained by welding several blanks with optionally different thicknesses, different materials, size and properties. At least theoretically, using this kind of technique the use of material may be optimized. Blanks of different thickness may be joined or a steel blank, may be joined with a coated steel blank for example, using the specific properties of each material where they are needed.
These blanks may be welded “edge to edge” (“butt-joining”). These so-called tailored blanks are designed to be hot stamped and afterwards be manufactured to form automotive parts. Tailored welded blanks may be used for structural components such as doors, B-Pillars, beams, floor, bumpers, etc.
Similarly, “patchwork” blanks are known, in which several blanks are not necessarily welded “edge-to-edge”, but instead partial or complete overlaps of blanks may be used.
An example of steel used in the automotive industry is 22MnB5 steel. In order to avoid the decarburization and the oxide scale formation during the heating and forming process, 22MnB5 is presented with an aluminum-silicon coating. Usibor® 1500P and Ductibor® 500P, commercially available from Arcelor Mittal, are examples of steels used in tailored and patchwork blanks.
Patchwork blanks and tailored blanks may also be used or useful in other industries.
Usibor® 1500P is supplied in a ferritic-perlitic condition. The mechanical properties are related to this structure. After heating, hot stamping, and subsequent rapid cooling (quenching), a martensitic microstructure is obtained. As a result, maximal strength and yield strength increase noticeably.
The composition of Usibor® 1500P is summarized below in weight percentages (rest is iron (Fe) and unavoidable impurities):
CSiMnPSCrTiBN0.240.271.140.0150.0010.170.0360.0030.004
As mentioned before, Usibor 1500P is supplied with an aluminum-silicon (AlSi) coating in order to prevent corrosion and oxidation damage. However, this coating has a significant effect related to its weld behavior. If Usibor 1500P blanks are welded without any further measures, aluminum of the coating may enter into the weld area and this can cause an important reduction of the mechanical properties of the resulting component and increase the possibility of fracture in the weld zone.
In order to overcome this problem a method was proposed in DE202007018832 U1 which consists in removing (e.g. by laser ablation) a part of the coating in an area close to the welding gap. This method has the disadvantage that an additional step is needed for the production of the (tailored) blanks and components and that in spite of the repetitive nature of the process this additional step requires a complex quality process with an elevated number of parts which are to be scrapped. This entails an increase of the cost of the welding step and limits the competitiveness of the technology in the industry. US20080011720 proposes a process for laser welding at least one metal workpiece by a laser beam, said workpiece having a surface containing aluminum, characterized in that the laser beam is combined with at least one electric arc so as to melt the metal and weld said workpiece(s). The laser in front of the arc allows the use of a flux-cored wire or the like containing elements inducing the gamma-phase (Mn, Ni, Cu, etc,) favorable to maintaining an austenitic structure throughout the melted zone.
US2014027414 discloses a hybrid welding system including a hybrid welding apparatus and methods of welding. The hybrid welding apparatus includes a laser, an electric arc welder with a non-consumable electrode and a wire feeding device. The electric arc welder provides an electric arc without feeding a welding wire. The wire feeding device is arranged and disposed to feed a wire to a treatment area, which is located between the projections of laser beam and the electric arc. The laser and the electric arc welder are arranged and disposed to direct energy toward at least two adjacent components to form a common molten pool.
US2013043219 discloses a method and a system to weld or join workpieces employing a high intensity energy source to create a weld puddle and at least one resistive filler wire which is heated to at or near its melting temperature and deposited into the weld puddle.
EP2511041 discloses a hybrid welding apparatus including a system and method for welding at least two adjacent components having a large gap of approximately 3.0 millimeters that results in a full-penetration weld. The welding system includes a hybrid welder having a defocused laser beam, an electric arc welder, and at least one bridge piece adjacent to one or more of the at least two adjacent components. The defocused laser beam and the electric arc welder are arranged and disposed to direct energy onto the at least two adjacent components to create a common molten pool operable to provide a full penetration weld to bridge the gap at a high constant weld speed, thereby joining the two adjacent components with a weld.
However, problems related to the only partial dilution of the filler materials along the depth of the welding zone have been found which result in a reduced welding strength.
Herein a blank may be regarded as an article which has yet to undergo one or more processing steps (e.g. deformation, machining, surface treatment or other). These articles may be substantially flat plates or have more complicated shapes.
In examples of the welding methods described herein the aforementioned disadvantages are avoided or at least partially reduced.